kernel - Fix pager bug in vm_fault and UFS and have UFS use vop_stdgetpages

kernel - Fix pager bug in vm_fault and UFS and have UFS use vop_stdgetpages

* UFS must call vtruncbuf() before b*write()ing the buffer related to
  a shortened file's truncation point.  vtruncbuf() can invalidate pages
  which are part of the buffer cache buffer.  The b*write() revalidates
  those pages.  If we fail to revalidate the pages we can end up with a
  buffer containing invalid pages with B_CACHE set.

* UFS was using its own getpages code in certain cases.  Adjust it to
  use vop_stdgetpages() in all cases by default.

  vop_stdgetpages() uses UIO_NOCOPY VOP_READ()s to fill in missing data
  and to perform read-ahead operations.  It should properly deal with
  partially valid and partially dirty pages as well as deal with file
  holes.

* vm_fault can pass a valid page to the readrest: code if PG_RAM is set.
  We cannot free this page if the underlying vnode contains a hole at
  that location.  Call vm_pager_get_page() unconditionally.

  This fixes fsx failures for both NFS and UFS (when UFS is using
  vop_stdgetpages()).